In general, a silicon single crystal has been manufactured by the Czochralski method (the CZ method) using a vitreous silica crucible. Silicon melt obtained by melting polycrystalline silicon raw material is retained in the vitreous silica crucible, and a seed crystal of a silicon single crystal is dipped into the silicon melt and the seed crystal is gradually pulled while rotating the seed crystal. The seed crystal acts as a core to grow a silicon single crystal.
Such a vitreous silica crucible has two-layer structure having an outer layer containing a number of bubbles, and a transparent inner layer. The property of the surface of the inner layer, that is, the inner surface contacting silicon melt during pulling a single crystal affects the property of an obtained silicon single crystal, and thus affects the yield of silicon wafers, which are the final product. Thus, conventionally, the inner layer is made of synthetic vitreous silica formed by amorphous synthetic silica powder, and the outer layer is made of natural vitreous silica.
Conventionally, in melting silicon and pulling a single crystal, there has frequently occurred a problem of melt surface vibration. According to this problem, dipping a seed crystal is disturbed by a wave generated on a liquid surface of silicon melt, and thus pulling of a silicon single crystal is prevented or single crystallization is deteriorated. This melt surface vibration phenomenon has become more likely to occur along with increase in diameter of a silicon crystal, the demand to improve the inner surface property of a vitreous silica crucible has become stronger.
The vitreous silica crucible used for the above-mentioned CZ method can be obtained by the processes of depositing silica powder inside a mold to form a silica powder layer, and fusing the silica powder layer by arc discharge, followed by cooling and solidifying. In such a method of manufacturing a vitreous silica crucible, when the silica powder layer is fused, the inner surface is cleaned by so-called fire polishing which is a process of performing arc discharge toward the inner surface of the silica powder layer. The fire polishing is a process of removing, by arc, bubbles generated in the fused material while forming a silica powder layer by fusing silica powder by arc discharge. It is possible to manufacturing a vitreous silica crucible having a good inner surface property by performing such fire polishing treatment.
However, when a vitreous silica crucible is manufactured by use of conventional linear electrodes, for example, as shown in FIG. 8, it is difficult to apply the arc from the electrodes 113 toward the whole area of the sidewall inner surface 111b of the silica powder layer 111. In this case, bubbles are uniformly removed on the bottom inner surface 111a by fire polishing. However, bubbles are not effectively removed on some part of the sidewall inner surface 111b. 
In this occasion, for example, arc discharge from linear electrodes 113 is directly applied toward the bottom inner surface 111a, and the silica powder layer 111 is effectively fused on this portion. However, when the conventional electrodes as shown in FIG. 8 are used, arc is difficult to be applied to the sidewall inner surface 111b and a curved surface, and radiation heat is also difficult to be applied thereto. Therefore, on some portion of the sidewall inner surface of a manufactured vitreous silica crucible, bubbles generated in the fusing material are solidified and appear on the surface. This deteriorates the inner surface property.
Then, when a silicon single crystal is pulled by above-mentioned CZ method by use of a vitreous silica crucible having property defects on the sidewall inner surface, single crystallization is prevented on the defects in the crucible, and this leads to decrease in the yield.
Furthermore, nowadays, in order to obtain a large-diameter wafer having a diameter of 762 to 1016 mm (30 to 40 inches), there has been a demand of increase of the diameter of a silicon single crystal, and this requires the size increase of the vitreous silica crucible. For this reason, when a vitreous silica crucible is manufactured, the amount of power necessary for fusing a silica powder layer increases, and thus the power applied to the electrodes is required to increase. In addition, it is required to uniformly apply arc discharged from the electrodes to the whole area of the inner surface of a silica powder layer having a large area. However, as the size of the vitreous silica crucible increases, the surface area of the crucible inside increases. Thus, it becomes more difficult to uniformly apply arc toward the entire inner surface of the sidewall. Then, the above-mentioned bubble removal by fire polishing becomes incomplete, and thus the inner surface property of the manufactured vitreous silica crucible substantially deteriorates. Then, when a silicon single crystal is pulled by use of a vitreous silica crucible having defects in the inner surface property, there can occur a problem in the growth of a silicon single crystal.
In order to avoid generation of property defects in the inner surface in the manufacturing process of a vitreous silica crucible, there is known a technique to remove impurities or the like generated in an apparatus by removing silica fume generated from fused silica powder material as in the technique described in, for example, Patent Document 1.
Furthermore, in order to further improve the inner surface property of a vitreous silica crucible, it has been proposed a method of using amorphous synthetic silica powder as silica powder to form the inner surface of the vitreous silica crucible as in the techniques of Patent Documents 2 and 3.